For the recovery of valuable metals from alloy scrap, a range of oxidative metallurgical and hydrometallurgical processes have been developed, but all of them have predominantly the disadvantage of a high energy consumption (for example roasting at high temperatures) and/or require the use of environmentally unfriendly reagents.
A good summary of the various processes is provided by the publications Kenworthy et al., “Experimental Extraction of Strategic Components . . . ”, Report of Investigations 5786, United States Department of the Interior, Bureau of Mines, 1976, and K. Vadasdi, “Effluent Free Manufacture of Ammonium Paratungstate (APT) by Recycling the Byproducts”, Int. J. of Refractory Metals & Hard Materials 13 (1995) 45-59.
Also mentioned in these publications are, electrolytic processes for the decomposition of alloys For energy-related and ecological reasons, electrolytic processes are generally preferable to the other processes. The advantages of the electrolytic decomposition processes for scrap are generally due to the elegant and streamlined process control, which, with current efficiencies of around 100%, ensures low process costs and also constitutes a better choice in environmental terms.
The major disadvantage of the electrolysis processes becomes apparent when it is necessary to decompose scrap which is present in powder form, typically with particle sizes smaller than 1000 μm, with average particle sizes of from 10 to 500 μm and preferably up to 200 μm, with the particle sizes of as little as 1 μm being included. In addition to the fundamental problem of handling such powders in an electrolysis cell, bringing a powder particle into contact with the anode, other problems arise, in particular, that of susceptibility of such powders to surface passivation, which leads to almost complete termination of the anodic oxidation processes. Consequently, within short electrolysis times, an uneconomically high increase in the electrolysis voltage takes place, together with the breakdown of water. Although for the electrolysis of scrap in large pieces, it is possible to counteract such passivation effects by suitable measures involving the electrolyte composition, pH adjustment, and current polarity reversal (J. Electrochem. Soc. India, 1986, 35-2, 127), in the case of powder batches, however, boundary-layer effects with pronounced pH gradients owing to low electrolyte mobility have a dominant effect and terminate the intended reaction.
It was an object of the present invention, while taking economic aspects into account, to provide an electrolytic process for the decomposition of powders and an electrolysis cell which is suitable for the electrolysis of powders.